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I think what poses a big problem (which I haven't mentioned in my original post) is that there are quite a few activities with only half the class present, which either have to be at the same time as an activity with the other half of the class and another teacher or in the first or the last timeslot of the day. FET usually filled the mornings fine, but often for only half the students.

Also you mentioned removing the noon time slot would be faster. I'm thinking about adding another timeslot for the long break in the middle of the morning, because one of our teachers has to change buildings for certain classes. The change schouldn't happen when there's only a five minute break. Do you think that's a bad idea?

I just found a trick myself: I simply added a pseudo activity at noon with every student. Now they have to stay in school until then, since there are no gaps allowed.

I am currently working on a timetable for a primary school. We are not allowed to let children go home before noon, so I would like to add a constraint, that doesn't allow students to leave early. I already added a constraint "Max gaps per week for all students". The problem is the two days where they have a free afternoon.
I also tried the constraint "A set of activities has a set of preferred time slots" and set it to 95% but now the timetable generation takes a very long time (I haven't yet tried if it actually finishes).

Is there any trick to have compulsory hours for all students?